Radar sensors are used in automotive engineering for measuring the distance to objects and/or the relative speed with respect to such objects outside of the motor vehicle. Examples of objects include preceding or parked motor vehicles, pedestrians, bicyclists, or devices within the vehicle's surroundings. The pulse radar functions, for example, at 24.125 GHz and may be used for the following functions, stop & go, precrash, blind spot detection, parking assistant, and backup aid.
FIG. 1 shows a schematic representation of a conventional radar system having a correlation receiver. A pulse generation 302 causes a transmitter 300 to transmit a transmission signal 306 via an antenna 304. Transmission signal 306 hits a target object 308, where it is reflected. The receiving signal 310 is received by antenna 312. This antenna 312 may be identical to antenna 304. After received signal 310 is received by antenna 312, the signal is transmitted to receiver 314 and subsequently supplied via a unit 316 having low pass and analog/digital conversion to a signal evaluation 318. The special feature of a correlation receiver is that receiver 314 receives a reference signal 320 from pulse generation 302. Receiving signals 310 which are received by receiver 314 are mixed in receiver 314 with reference signal 320. As a result of the correlation, conclusions may be drawn, for instance, as to the distance of a target object, on the basis of the temporal delay between the transmitting and receiving of the radar impulses.
A similar radar device is described in German Patent Application No. DE 199 26 787. In this context, a transmission switch is switched on and off by the impulses of a generator so that a high-frequency wave generated by an oscillator and conducted via a termination hybrid to the transmission switch is switched through to the transmission antenna during the pulse duration. A reception unit also receives the output signal of the generator. The received signal, i.e., a radar pulse reflected by an object, is combined with the oscillator signal, which reaches the mixer via a reception switch, and evaluated during a predefined time window.
U.S. Pat. No. 6,067,040 also uses a transmission switch that is switched on and off by generator impulses. Separate paths for I and Q signals are provided for reception of the reflected radar pulses. Also in this instance, the received signal is only mixed and evaluated during a predefined time window. Both in the radar device according to DE 199 26 787 and in U.S. Pat. No. 6,067,040, the generator signal first reaches a reception switch/pulse modulator.